1. Field of the Invention
The present invention relates to a photometric device in which responsive characteristics of a light receiving element is improved, and particularly to an improvement of responsive characteristics of a photometric device suited for a camera.
2. Description of the Prior Art
Generally speaking, light from an object has a wide range of wave length zone and spectral sensitivity of such light receiving element as a silicon photo-diode, etc. has such characteristics as shown by E.sub.1 in FIG. 1. But a wave length zone which can actually be visually sensible by a naked eye is 400 m.mu. to 700 m.mu.. Therefore if light from an object, as it is, is sensed, light in infrared zone greatly affect the sensing thus an appropriate exposure cannot be obtained.
Therefore, a filter for compensation of spectral sensitivity is provided in front of a light receiving element to eliminate the effect of the infrared light on the light receiving element so that a compensation is made to such spectral characteristic as close to a relative luminosity in a conventional method.
However, in a conventional photometric circuit in which a photo-electric conversion element of a photo-voltaic type is used as a light receiving element, an operational amplifier with high input impedance having at its input stage a field effect transistor, FET, is connected to said light receiving element, therefore, unnecessary charging is instantaneously made to such stray capacitance as a capacitance existing at the field effect transistor FEI as a light source is put in, a junction capacitance of said light receiving element, and a capacitance generated at a printed circuit network, etc., and a considerable length of time is required to discharge said unnecessary charge, thus a photometric circuit is difficult to be stabilized. This stands out particularly as an amount of photo-current is small when a photometry is made for a weak light beam.
Further, since photo-current to a logarithmic conversion diode inserted in a negative feedback circuit is cut off in a black out state in a conventional photometric circuit, negative feedback will not work on an operational amplifier with high input impedance, and an output of said amplifier will have either a high level or a low level depending on a direction of the diode. That is, the amplifier is latched. Therefore, it takes time to release said latching on the operational amplifier at a time of next photo-taking, thus follow-up characteristics of a photometric circuit to an amount of incident light becomes poor.
An object of the present invention is to eliminate said shortcomings, and is to provide a light source emitting light with a prescribed wave length zone between a light receiving element and a filter for compensation of spectral sensitivity which is positioned in front of said light receiving element, so that responsive characteristics of the light receiving element and responsive characteristics of the photometric circuit are improved.
Another object of the present invention is to control light emission of a light source provided between a light receiving element and a compensation filter.